This invention relates to the transportation and/or storage of liquified and/or compressed gases in bulk cargo tanks and the like. More particularly, it is concerned with an improved system for mounting a cryogenic liquid holding tank of aluminum metal construction within and integrating such tank with an outer envelope or supporting hull structure of ferrous metal.
Thoughout the world today there is an ever increasing need and demand for natural gas as well as inexpensive and efficient means for storing and transporting this gas in the form of liquified natural gas (LNG) from one location to another. During such transport in the liquid state, the natural gas is generally held at a temperature of about -260.degree.F. at approximately atmospheric pressure.
The relatively low temperatures at which liquified gases, such as liquified natural gases, must be kept during transport in combination with the varied stresses to which the transport tanks themselves are subjected during movement have posed numerous problems to the tank builder.
For example, severe static stresses are imposed on shipboard tanks due to the extreme temperature variations that occur in the tank during loading and unloading. Severe dynamic stresses are set up in the shipboard tank structures due to the liquid cargo accelerations and the sloshing of the liquid cargo in the tanks during ocean transport as well as from the deflections and bending of the transport vessel itself when moving through heavy seas. Thus, in contrast to situations where simple, conventional, shore-based tank support systems might be used for on-shore storage and transport, marine transport systems for liquified gases had to be substantially altered in order to withstand the numerous and varied severe stresses imposed upon a vessel and its cryogenic cargo tanks during an ocean voyage or the like.
At the present time, there are four principal tank containment systems used on cryogenic transport vessels. They are commonly referred to as "prismatic free-standing tanks," "spherical free-standing tanks," "semi-membrane tanks," and "membrane tanks." These various tank structures and the vessels in which such tank structures are incorporated as well as the individual merits of each are described in considerable detail in a paper which was presented by William DuBarry Thomas et al to the Society of Naval Architects and Marine Engineers on Nov. 11-12, 1971, and the title of this paper is "LNG Carriers - The Current State of the Art."
When the containers or tanks for certain of these systems are made of metal adapted to be in direct contact with the cryogenic liquid, they have to be formed from materials which are not subject to brittleness failure at low temperatures, such as, for example, aluminum, stainless steel or 9 nickel steel. Aluminum has been preferred because of its cost and the steel ordinarily used in tank construction has not been used because of its susceptibility to embrittlement at the very low liquified gas holding temperatures.
When any of the four above-noted containment systems were installed on a vessel, a complicated arrangement has been provided for appropriately insulating and isolating the tank from the ship's hull or bulkhead lest the low temperature cryogenic liquid possibly crack or embrittle the customary steel plates of the basic hull structure of the vessel. Various types of insulating materials, such as perlite, PVC foam, polyurethane foam, fiberglass or various combinations thereof have been used either to line the inner walls of the cryogenic tanks or for emplacement between the upstanding inner cryogenic tank walls and the bulkheads of the inner hull in order to thermally isolate the cryogenic tank from the hull structure. These expedients have been costly and have not always made the most efficient application of the materials used.